1. Field of the Invention
The present invention relates to a method for forming a photomask pattern in which an integrated circuit pattern is formed on a semiconductor substrate, and more particularly to a method for forming a phase shift mask pattern at high precision.
2. Related Background Art
In the fabricating of a semiconductor device represented by a semiconductor integrated circuit (thereinafter abbreviated as LSI), a photomechanical process referred to as a so-called photolithography technique is used to form an integrated circuit pattern on a semiconductor wafer efficiently.
This photolithography technique uses a preformed photomask to form a pattern for the diffusion area of impurities, wiring pattern or contact hole on a wafer, whereby a corresponding pattern of the photomask is optically transferred onto the wafer.
Conventionally, a light shielding film pattern made of chromium formed on a transparent substrate made of quartz was used as the photomask. However, with the higher integration of LSI and the finer formation of pattern on the wafer, an attempt to improve the resolution of a transfer pattern has been made by varying the configuration of the photomask.
One such attempt involves a phase shift mask. The phase shift mask is one in which in addition to a conventional pattern consisting of a light shielding film region and a light transmitting region, a pattern consisting of a so-called phase shifter is formed on the mask surface. The phase shifter has a function of changing the phase of transmitted light, whereby a phase shift mask having a higher resolution limit than conventional masks can be obtained, even with the same projection lens, by forming a mask with an appropriate design of the phase of this phase shifter and the positional relation between the light shielding film pattern and the light transmitting pattern.
Currently, there have been proposed the following three types of structures for the phase shift mask (see Nikkei Microdevices, July, 1990, pp. 108-114, and April, 1991, pp. 75-77).
(1) Spatial frequency modulation type
(2) Edge intensified type
(3) Light shielding effect exaggeration type
Among them, the methods (1) and (3) have several well-known problems in the practical use for semiconductor fabrication process, such that the arrangement of a phase shifter pattern may be difficult depending on the shape of the pattern to be formed, and only the negative type of photo resist can be used in the transfer of the pattern. On the contrary, the method (2) allows the arrangement of a phase shifter for a pattern of any shape, because of its mask configuration in which the phase shifter pattern is provided on only a boundary portion between the light transmitting pattern and the light shielding film pattern. Also, it has a number of advantages in the practical use in that the photo resist of either positive or negative type can be applied.
One of the formation methods for the phase shifter mask is such that after the light shielding film pattern is formed on the surface of a mask substrate, a transparent film is formed all over the surface of the mask substrate, and left behind only on the edge portion of the light shielding film, with the lithography method, to thereby make a phase shifter.
Another method is provided in which after the photo resist is applied over the surface of a substrate having the light shielding film pattern formed thereon, a photo resist mask is formed in a self-alignment manner only on the light shielding film with the development through ultraviolet light exposure over the entire surface from the back face, and then the anisotropic etching is made on the surface of the substrate to remove the substrate surface down to an intended depth. Then, only the light shielding film pattern remaining between the photo resist mask and the substrate is etched isotropically, and side-etched intentionally to an intended width, and then the photo resist mask is removed. In this case, a step portion formed by etching the substrate is a phase shifter.
This latter method has more advantages than the former method, in that although the number of lithography processes is increased by one in creating the mask, the resist mask for the formation of phase shifter can be formed in a self-alignment manner, and no addition of the film formation process for phase shifter is needed.
In the former method in which a transparent film is formed on the entire surface of a mask substrate having the light shielding film pattern formed thereon, and is made a phase shifter with the lithography method, the film formation process and the lithography process are performed twice respectively in creating the mask, and it is difficult to secure a sufficient alignment precision between the light shielding film pattern and the phase shifter pattern over the mask surface, since an electron beam exposure apparatus for use with the mask formation has normally no function corresponding to the alignment feature which is provided in an optical exposure apparatus.
In the latter proposal, since the resist mask used in the anisotropic etching of mask substrate is directly used as an etching mask in the side etching of the light shielding film pattern, it is impossible to ignore the influence from the deterioration of the side wall protective film or resist formed in the anisotropic etching, and the variation in size, whereby there are problems associated with the 10 precision of side etching, the reproducibility and the occurrence of many defects.
As above-described, with the heretofore proposed methods, it was difficult to improve the precision of forming the phase shifter pattern, and therefore it was extremely difficult to realize the resolution of the phase shift mask as designed.